Total stations are precision survey instruments that allow a user to electronically measure the distance, as well as the vertical and horizontal angles, between the total station and any points to be surveyed. These types of measurements require a high degree of accuracy, with acceptable tolerances measured in arc-seconds for angles and millimeters for distance. In addition to the demand for precision, it is also important that these instruments be easy and convenient to use, because of the difficult terrain and other extreme conditions that may be present in the location where the total station is to be employed.
When using a total station, it is necessary for the user to precisely level the instrument within a manufacturer-specified range before measurements are taken. In the prior art, electronic tilt sensors are a well-known and commonly used means of establishing whether the total station, in particular the rotational alidade portion of the total station, is level. The tilt sensors are typically integrated into the total station, and the data from the tilt sensors are visually displayed to the user on a screen present on the total station.
In one prior art total station, an electronic tilt sensor is comprised of a vial filled with fluid. The fluid in the vial is free to move, so that if the tilt sensors are not level, the level of the fluid in the vial is different from the fluid level if the tilt sensors are level. In another prior art total station, an electronic tilt sensor includes elements suspended by wires that are used to determine the relationship between the vertical axis of the instrument and the true vertical line. The amount of tilt is measured in arc-seconds. Typical electronic tilt sensors are capable of measurements within one (1) arc-second of angle.
The tilt sensor may be affected by the ambient conditions where the total station is being used. Thus, a bias of unknown quantity may be introduced into the level measurement provided by the tilt sensors if the ambient conditions at the survey site are significantly different from those present when the tilt sensors were calibrated. There may also be biases introduced due to other factors. Because of the demand for precision, it is necessary to quantify the total bias in the tilt sensors and account for it when the rotational alidade portion is leveled. To account for the ambient effects, the tilt sensors are calibrated at the site where the total station will be used.
The bias in the tilt sensors is quantified using a known procedure. The known procedure involves measuring the angle of tilt twice. After the total station is set up, the rotational alidade portion is set at an initial rotational orientation and the angle of tilt is measured. The rotational alidade portion is then rotated approximately 180 degrees from its initial position and a second measurement of the angle of tilt is taken. Because the rotational alidade portion rotates in a fixed plane, the average of the values of the two measurements provides the bias in the tilt sensors. If the sensors are perfectly level with respect to the bearings in the base of the rotational alidade portion, then the average of the two measurements will be zero, provided the instrument is level within the range of the sensors. Once the bias is known, the tilt sensors can be calibrated to account for the bias and the true angle of tilt can be determined. The rotational alidade portion is then leveled according to the true angle of tilt.
After the rotational alidade portion has been leveled, it may subsequently deviate slightly from the level position. For example, uneven thermal expansion in the legs of the tripod upon which the total station is mounted may cause the total station, and consequently the rotational alidade portion, to tilt slightly. As discussed above, the acceptable tolerances for measurements taken using a total station are very tight, and thus even a very small degree of tilt introduced after the total station has been initially leveled must be accounted for.
With reference to Prior Art FIG. 1A, total station 100 and rotational alidade portion 102 are tilted in the horizontal direction; that is, they are leaning slightly forward on inclined horizontal plane 110. Inclined horizontal plane 110 forms horizontal angle of inclination .theta. relative to and below true horizontal plane 120.
With reference now to Prior Art FIG. 1B, after rotational alidade portion 102 has been rotated 180 degrees from its initial rotational orientation, rotational alidade portion 102 is now pointing slightly upward, again forming horizontal angle of inclination .theta. relative to true horizontal plane 120, with horizontal angle of inclination .theta. now above true horizontal plane 120. With rotational alidade portion 102 rotated less than 180 degrees from its initial orientation in either direction, there will still be a horizontal tilt, although the horizontal angle of inclination will be less than .theta. (rotational alidade portion 102 will pass through one point where horizontal angle of inclination .theta. is zero). Note that the angle between the axis of rotation of rotational alidade portion 102, designated by 130, and true vertical plane 135 is also the angle .theta..
With reference to Prior Art FIG. 2, prior art total stations are equipped with a tilt compensator to compensate for the degree of tilt based on the data from the tilt sensors. Thus, it is not necessary for the user to continuously re-level the total station prior to each measurement. In the prior art, the horizontal angle of inclination is measured by the tilt sensors at each new rotational orientation of the rotational alidade portion. In the prior art, at each new rotational orientation the tilt compensator monitors the horizontal angle of inclination determined by the tilt sensors, calculates a compensation factor, and applies the compensation factor to correct the measurements taken with the total station. The measurement results are then displayed to the user of the total station on screen 222 located on rotational alidade portion 208. The measurement results are displayed in units of degrees 223, minutes (arc-minutes) 224, and seconds (arc-seconds) 225.
As discussed above, the tilt sensors in the prior art are comprised of either a vial filled with fluid or elements suspended by wires. Hence, when the rotational alidade portion is rotated to a new orientation, the acceleration forces created by the rotation cause the fluid or the suspended elements in the tilt sensors to move back and forth. In other words, after the rotational alidade portion is rotated to a new orientation, the fluid or suspended elements within the tilt sensors are not stable. In the prior art, a period of time must pass before the fluid or suspended elements return to the equilibrium condition that allows there to be an accurate measurement of the amount of tilt of the rotational alidade portion.
Therefore, one disadvantage to prior art total stations is that there is a time lag between the time the rotational alidade portion is rotated and the time that the tilt sensors are capable of providing an accurate measurement of the degree of tilt at the new rotational orientation. Thus, in the prior art, there is also a time lag before the tilt compensator can be accurately used. In the prior art, the user of the-total station is therefore inconvenienced after the rotational alidade portion is rotated because he or she must wait until the tilt sensors have reached equilibrium before proceeding with the desired measurements. In addition, other work may be held up while waiting for the results of survey measurements thereby increasing labor costs.
Another disadvantage to prior art total stations is that survey measurements taken before the tilt sensors reach equilibrium are only approximate, because the tilt compensator does not have an accurate measurement of the horizontal angle of inclination and so cannot calculate a precise compensation factor. Hence, in the prior art, the smallest units of measure that are typically available until the tilt sensors reach equilibrium are units of degrees. In one prior art approach, with reference again to FIG. 2, minutes 224 and seconds 225 are displayed, but rapidly scroll from one value to another as the measured values of minutes and seconds increase and decrease while the fluid or suspended elements in the tilt sensors oscillate back and forth. In another prior art approach, the display of minutes 224 and seconds 225 are left blank until the tilt sensors reach equilibrium and the degree of tilt is able to be accurately measured. Thus, another disadvantage to the prior art is that precise measurement results cannot be obtained or displayed to the user of the total station until the tilt sensors have reached equilibrium, again delaying and inconveniencing the user.
Thus, a need exists for a total station that allows the degree of tilt of the rotational alidade portion to be quickly determined after the rotational alidade portion has been rotated to a new orientation, without the time lag that occurs before the tilt sensors reach equilibrium. Another need exists for a total station that meets the above need and also displays precise measurement results to a user without the aforementioned time lag, so that the user is not further inconvenienced. Still another need exists for a total station that meets the above needs and also automatically determines the degree of tilt and applies it in the total station tilt compensator. A still further need exists for a total station that meets the above needs, and is user-friendly and compatible with the current practices and training of those who will be using the total station.